The present invention relates generally to the field of electromechanical pressure transducers and more particularly to the field of ceramic capacitive pressure transducers.
Ceramic capacitive pressure transducers are known and used in automotive systems for sensing engine vacuum pressure. Generally they comprise parallel plate capacitor electrodes separated by a gap wherein the spacing between the parallel plate electrodes is altered in response to sensed pressure changes thereby changing the capacitance created by these electrodes. Typically, one capacitor electrode is deposited on a top end surface of a relatively thick cylindrically shaped ceramic base substrate while the other capacitor electrode is deposited on a relatively thin disc shaped ceramic diaphragm. An annular glass insulating ring is deposited on the base substrate and provides a mounting means for joining the diaphragm to the base substrate as well as providing a sealing means for forming an internal cavity defined by the diaphragm, the top base substrate surface and the insulating ring. A predetermined reference pressure exists in the internal cavity and a predetermined displacement of the diaphragm with respect to the top surface of the base substrate is provided in response to the pressure difference between the pressure within the internal cavity and the pressure external to the internal cavity. An example of such a pressure transducer is illustrated in U.S. Pat. No. 4,178,621 which is assigned to the same assignee as the present invention.
One problem which exists with capacitive pressure transducers similar to those described above concerns the providing of a strong and reliable electrical and mechanical connection between the output leads of the transducer and the capacitor electrodes. In U.S. Pat. No. 4,178,621, noted above, the electrode output leads pass through holes in the base substrate and the electrical and mechanical connection of the leads to the respective electrodes and the diaphragm and base substrate is provided through the use of conductive epoxy. The capacitive transducers manufactured in accordance with the teachings in the above noted patent have the conductive epoxy applied directly to the transducer electrodes and leads in areas of overlap between the diaphragm and the base substrate. This electrically and mechanically connects the leads in the same manufacturing operation. While this technique is commercially feasible and has been used in the past for production units, this technique does not permit adequate visual inspection of the electrical and mechanical bond between the output leads and the transducer electrodes. In addition, since a single material, conductive epoxy, is utilized to provide both the electrical and mechanical bond between the output leads and the electrodes, as well as providing the mechanical bonds which attach the output leads to the transducer assembly, problems have occurred due to mechanical stresses on the leads rupturing the electrical and/or mechanical connections between the output leads and the electrodes since the conductive epoxy may not provide a strong enough mechanical bond for the leads.
In some prior transducers an annular ring of conductive metallization is provided on the base substrate top surface to contact the diaphragm electrode and thereby provide metallization on the top surface directly connected to the diaphragm electrode, while this ring also performs the function of sealing the internal cavity. This dual use of the ring requires compromising the mechanical and electrical properties of the ring. This type of structure also requires a metallization cross over in order to bring a connection to the base substrate electrode outside of the perimeter of the ring to enable connecting a lead to the base electrode. This adds to the complexity of the sensor by requiring additional insulating layers. Also, the use of this type of technique for the sealing ring would generally require the use of solder for providing the seal, and this may not be desirable in many instances.